Quick change tool holders are widely used in industry for various applications. One such quick change tool holder in wide use today is the KM series manufactured by Kennametal Inc. of Latrobe, Pa. For an appreciation of the KM tool holder one is referred to U.S. Pat. Nos. 4,836,068; 4,747,735; 4,736,659; 4,726,269; 4,723,877; and 4,708,040. The KM tool holder series utilizes a ball locking system to hold a tool holder in a tool block. In particular, the tool holder includes a shank having one or more apertures formed therein. A threaded locking rod extends through the central portion of the tool block and holder assembly. Formed on the locking rod is a ramp that is effective to engage one or more locking balls disposed between the locking rod and the aperture or apertures of the shank. To lock the tool holder within the tool block, the locking rod is turned such that the ramp engages the locking ball or balls and urges the same outwardly into locking engagement with the aperture or apertures formed in the shank. To unlock the tool holder from the tool block, the locking rod is rotated in the opposite direction allowing the balls to roll down the ramp and out of the locking position with the aperture of the tool holder shank. Hence, it is the axial movement of the lock rod and ramp structure that results in an inward disengagement or an outward engagement of the balls with the tool holder. This method requires several rotations of the lock rod to obtain the amount of axial movement necessary to fully engage or disengage the tool holder.
In order to facilitate quicker and more efficient engagement and disengagement of tool holders, Krupp Widia has developed a cam lock mechanism which is disclosed in European Patent Application No. 0369211. In this design a cam and screw assembly, which includes a threaded shaft and a cam lock surface located on the remote end of the threaded shaft, drives the balls into a locking position with the shank of the tool holder and maintains the lock. As the cam and screw assembly is rotated, it is not the axial movement of the mechanism that locks or disengages the balls, but instead the rotational movement of the cam structure. To lock the tool holder with the shank, the cam is rotated by a screw and in the process the cam surface engages one or more locking balls and drives them outwardly into locking engagement with the shank of the tool holder. In an unlocked or disengaged position, the balls rest in recessed portions of the cam structure. The angle of the cam maintains the ball or balls in an engaged or disengaged position as long as the cam remains in the selected position. This cam and screw assembly allows quicker and more efficient locking and release of the tool holder because only a turn of 180.degree. or less is required to fully engage or disengage the balls.
While there are many advantages to the cam and screw assembly design of Krupp Widia, in some applications there is a tendency for the cam and screw assembly to self-release. Where a conventionally threaded screw is used, the cam and screw assembly is sometimes "backed off" due to high forces being transmitted back through the balls to the screw. The resulting rotation of the screw results in an altered cam position and thereby a complete or partial disengagement of the balls. Due to the physical constraints of typical tool holders like the KM series, it is not practically possible to modify the angle of the cam to avoid such self-release.
Therefore, there is a need for an efficient locking mechanism for holding the cam in a locked and stationary position even while substantial forces are being translated through the cam back to the locking mechanism tending to cause the locking mechanism to be at least partially disengaged.